Daughter bugs are the bugs introduced as a result of fixing another bug.
Consider that Anna is testing requirement A and found a bug. She created a bug report and the bug id is Bug001.
Development team fixed the Bug001 and sent it to Anna for testing.
During Bug001 testing, Anna found a new bug due to Bug001 fix. She created a bug report and bug id is Bug002.
In this example, Bug002 is a daughter bug.